As for why he got them........ Well, some horses are just prone to them or it could have been anything from a bad trim one time to a tiny rock that got imbedded in the white line and worked its way up into the foot. He could have stepped on something sharp or it could have been a stone bruise or something that abcessed. A one time thing is not really something to be terribly concerned about; however, if he starts having them consistently, you might consult a vet.
BTW, do you have any new pix? I always adore pix of Thunder, he is such a beauty.